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It's interesting sometimes, the stories that get media traction.

This time it's the one about the tiny Samoan airline that has decided to charge fares based on a passenger's weight. The move has touched off discussions about whether such an idea makes sense for mainline carriers as well. After all, Americans are quite a bit larger than they used to be, and doesn't that extra weight affect an airplane's performance? Is it just a matter of time before passengers on United, Delta or American are asked to stand on a scale, like their suitcases, when checking in?

Well, no. It makes sense if, like Samoa Air, you're a company operating light, 10-seat aircraft in a region with extremely high rates of obesity, but not if you're a major carrier with a fleet of Boeings or Airbuses. There are some touchy social and civil liberty aspects that probably make the idea controversial, if not untenable, but let's take a look at the practical side:

To begin with, passengers account for surprisingly small percentage of a plane's overall weight. As a general rule, the larger the plane, the less of a factor it is. That sounds counterintuitive, but consider the example of a Boeing 747">Boeing 747. A 747 typically seats around 420 people, and its maximum allowable takeoff weight is 875,000 pounds. Four hundred and twenty people *and* their carry-on luggage weigh in at just under 80,000 pounds. That's less than 10 percent - 10 percent! - of the plane's maximum heft. Most of what it weighs comes from the mass of the jet itself, plus the fuel load, which runs into the hundreds of thousands of pounds.

Airlines use standard approximations for people and luggage. The values - currently 190 pounds per person, including carry-ons, and 30 pounds per checked bag - are adjusted slightly higher during winter to account for heavier clothing. (Please don't ask me about trans-climate routes, i.e. London to Cape Town or New York to Rio; I honestly don't know if the origin or destination season is used.)

After the crash of a 19-seater in Charlotte, North Carolina, in 2003, 10 pounds were added to the standard passenger weights, and five to the luggage. What happened in Charlotte was not the consequence of an overweight or out-of-balance condition, but it nonetheless drew attention to the topic, and the FAA responded by acknowledging our expanding waistlines. It's possible the numbers could be upped again in the not-too-distant future, particularly with passengers lugging aboard larger and heavier carry-ons than ever before.

Crews are schooled in the finer points of weight and balance calculations, but in practice, it's the planners, loaders, and dispatchers who crunch the numbers. The boarding tallies are added to something called the BOW - basic operating weight - which is a book value of the ship itself, replete with all furnishings, supplies, and crew. (This BOW is adjusted from time to time, such when components are removed or added.) Compounded with fuel and cargo, the result is the total gross "ramp" weight. Fuel used for taxiing is subtracted to reveal the takeoff weight.

Around the time of push-back or shortly thereafter, a detailed manifest is sent to the cockpit printer. It shows the passenger total, fuel and cargo totals, center of gravity information, and gives us all of the required takeoff speeds (V1, VR, V2) and flight control settings (how many degrees of flaps, the stabilizer trim setting, etc.), for each possible departure runway.

And yes, for many planes, especially bigger ones, the maximum takeoff weight is substantially higher than the limit for landing. Thus if a return or diversion is required, a certain amount of fuel will need to be burned away first. Or, on larger jets that are so equipped, it can be jettisoned overboard. An overweight landing will be made if need be, but unless things are urgent, we'll usually take the time to get within limits. Even on planes with a fuel dump system, this can take 30 minutes or more. (No, the fuel does not come splashing down on people. Unless in an urgent emergency, dumping takes place at relatively high altitude and the fuel dissipates long before reaching the ground.)

For a better sense of how much of a plane's poundage passengers account for, here are some comparisons of different aircraft models. Most takeoffs aren't made at maximum allowable weight, it's true, and neither are most planes fully occupied; passenger load factors average around 80 percent. For simplicity, however, these examples assume max takeoff weight and every seat taken. Weight limitations can vary plane to plane, as can seating configurations; these are typical totals:

Little Samoa Air, who got all of this started, uses a twin-engine propeller plane called a BN-2 Islander (like the one I took to Kaieteur Falls, in Guyana). The Islander maxes out at around 6,600 pounds and has room for nine passengers, accounting for up to 26 percent of the plane's total. In the case of a Cessna Caravan, a popular singe-engine utility transport used all around the world, it's 30 percent.

For the small commuter and air taxi outfits who operate such planes, the girth of its customers matters a lot more than it does to an airline flying 747s. Should you fly with one of these companies, that 190-pound standard no longer applies. Embarrassed or not, you'll be asked to divulge your exact weight, and your bags, too, will be weighed. This is nothing new. What's different about Samoa Air is that it's the first airline to actually charge higher fares to fatter passengers.

No, we're not going to see this in these parts, though it does stir up some provocative questions ...

Even if heavyset passengers can't much affect a jetliner's performance, there are definitely some implications when it comes to cabin safety and comfort. Maybe you've seen those seat belt extensions used on occasion? People are bigger than they used to be, but airline seats are not. Should passengers of a certain size be required to purchase two seats? Should they be prohibited from sitting in an exit row? Should airlines increase seat dimensions and legroom with larger customers in mind?

That last one is basically a nonstarter with profit margins as thin as they are (no pun intended). As for the rest, I really don't know what the best course of action might be. I certainly don't want to offend anybody, so perhaps these are topics (here comes more stupid wordplay) that I'd best not weigh in on.

About the author

Patrick Smith is an airline pilot, air travel columnist, author, and host of www.askthepilot.com. In his spare time, he has visited more than 80 countries and always asks for a window seat. He lives in Somerville.
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